1. Field of the Invention
The present invention relates generally to computer networks and, more specifically, to maximizing operability of links between packet network elements such as bridges, switches and routers.
2. Description of the Related Art
Packet-switched computer network devices, such as bridges, switches and routers, are connected to each other by physical links, such as cables, that are attached to the devices' physical ports. The links are managed by layer 2 of the well-known seven-layer communication protocol stack model, the Open Systems Interconnection (OSI) Basic Reference Model that defines the protocol stack layers as, from top or layer-7 (most abstract) to bottom or layer-1 (least abstract): Application, Presentation, Session, Transport, Network, Data link, and Physical. A layer in this context is a collection of related functions or logic that provides services to the layer above it and receives services from the layer below it. Examples of well-known layer-2 services or protocols specified by the Institute of Electrical and Electronics Engineers (IEEE) include link aggregation, spanning tree algorithm and protocol, and Connectivity Fault Management (CFM).
Link aggregation in the Ethernet context is defined by a standard promulgated by the IEEE, referred to as IEEE Std. 802.3™-2005, Clause 43 and Annexes 43A, 43B and 43C, and refers to the parallel grouping of two or more physical links such that a logical port treats them as a single logical link. The physical links are aggregated into a link aggregation group, such that a Media Access Control (MAC) client can treat the link aggregation group as if it were a single (logical) link. Stated another way, though the links of a link aggregation group may be attached to several different physical ports, they appear to the MAC client as attached to a single logical port. To this end, IEEE Std. 802.3-2005 specifies the establishment of Data Terminal Equipment (DTE)-to-DTE logical links, consisting of N parallel instances of full duplex point-to-point physical links operating at the same data rate. Link aggregation is typically controlled by logic residing in a Link Aggregation Sublayer (LAS) of layer-2 of the protocol stack logic.
A conventional (prior art) bridge 100 is illustrated in logic diagram form in FIG. 1. The illustrated bridge architecture is typical for a bridge that conforms to IEEE Std. 802.1D™-2004 and IEEE Std. 802.1Q™-2005, but the description below is generally applicable to any similar architectures based on these or any similar standards. The bridge 100 includes communication protocol stack logic 102, and four physical ports attached to four corresponding links 104, 106, 108 and 110. Of the various layers of protocol stack 102, only the Physical layer 112, the MAC layer 113, the LAS 114, and other entities such as optional entities 116 and other entities 118, are shown for purposes of clarity. Examples of optional entities 116 include CFM, as defined by the IEEE P802.1ag draft standard, and MAC Security, as defined by IEEE 802.1AE™-2006. Other entities 118 can generally communicate with each other via a MAC Relay Entity 120 and with higher-layer entities 122 via Logical Link Control (LLC) entities 124. Bridge 100 can communicate traffic with other network elements (not shown in FIG. 1) via links 104-110. (As used herein, the term “traffic” means a series of one or more packets comprising data or control information.) Note that although a link 104-110 can be either bidirectional or unidirectional, dual-ended arrows are shown in this example to indicate bidirectional links for purposes of illustration. Also note that any combination of links 104-110 can be aggregated, as indicated for links 106-110 by the broken-line oval around links 106, 108 and 110, for example. Thus, in this example, network element 100 treats physical links 106, 108 and 110 as a single logical link. Unless otherwise specified as ‘logical,’ any reference hereinafter to a link refers to a physical link.
As noted above, a physical link can be either unidirectional or bidirectional. In networks with higher volumes of unidirectional traffic than bidirectional traffic, it would be desirable to deploy equipment (e.g., bridges, switches, routers, etc.) that supports as few bidirectional links as possible because a deployment of network equipment that supports bidirectional links can be less economical than a similar deployment that supports a small number of bidirectional links and a higher number of unidirectional links. However, all layer-2 protocols must use bidirectional links to operate fully, i.e., for all features of the protocol to operate exactly in accordance with the standard that defines that protocol.
The present invention addresses these problems and deficiencies and others in the manner described below.